While not restricted thereto, this invention finds immediate application in simple, inexpensive battery charging circuits which are especially useful for charging batteries used in portable battery-operated devices such as soldering pencils, shavers, cordless telephones, calculators, computers, television sets, radios, recorders, electric garden tools, cordless hand tools including woodworking and metalworking tools, and the like. More particularly, the present invention finds direct application in battery charging circuits for cordless devices which are simple, inexpensive to make and may be readily incorporated directly into the cordless devices.
Battery-operated cordless devices, such as hand tools, have a considerable popularity because of the convenience afforded by these devices. Devices of this type are customarily provided with a small D.C. electric drive motor which is energized by one or more rechargeable batteries incorporated within the tool housing and operatively associated with the electric drive motor. When the device is a television set, radio, recorder or the like, the rechargeable batteries supply D.C. power to the circuits therein either exclusively or alternatively, in instances in which an A.C. power cord and plug are provided. The elimination of a conventional electric cord or its alternative use increases the freedom with which the user of the cordless device can move about, eliminating the inconveniences of clearing the power cord from obstacles with which it may otherwise become entangled and reducing the necessity of having electric wall outlets available at every place the device is to be used. It has become customary to provide cordless devices, such as cordless garden tools and woodworking tools with rechargeable batteries so as to avoid the necessity of frequent battery replacements, and to reduce the cost of operation. The present day rechargeable batteries, properly maintained with a special charge, have an extended life and greatly enhanced convenience with which the cordless device may be operated.
Most cordless devices, such as cordless hand tools and the like, operate from a D.C. battery potential of only a few volts. The prior art has resorted to recharging the batteries from a conventional 117 volt, 60 Hz electrical outlet through voltage stepped-down transformers which necessarily are of considerable size and consequently increase the size, cost and weight of the battery charger. Different outlet voltage levels and/or supply frequencies are also encountered. The size and weight of the battery charger, and particularly the contribution to the weight by the step-down transformer, has made it difficult to incorporate a charging circuit directly into cordless devices and to make such a charger inexpensively. Moreover, the handling, storage and packaging of battery chargers involving transformers result in serious shortcomings and limitation on their use, particularly if desired to be incorporated in the cordless device itself. Moreover, the customer and user of a variety of cordless tools, appliances or devices will accumulate an equal variety of separable chargers, and some confusion may result in determining which charger is to be used with a particular cordless device.
It is known from U.S. Pat. No. 3,943,423 to Philip A. Hoffman entitled "Battery Charging Circuit" and issued on Mar. 9, 1976 to provide a battery charging circuit which eliminates the need for a relatively bulky and heavy voltage step-down transformer, and which, when recharging batteries in a hand tool or the like, needs simply to be connected to a conventional, 117 volt 60 Hz household outlet and to the battery cell or cells which are to be recharged. Here again, other outlet voltage levels and/or supply frequencies can be used as well. The known charging circuit of the aforesaid Hoffman patent comprises a variable resistance switch preferably realized in the form of a PNP junction transistor and Darlington-connected other transistors operatively associated with a feedback circuit. This known circuit has, in addition to the transistors and resistors, two rectifying diodes and two capacitors, resulting in a circuit which, particularly because of the need for the capacitors and a considerable number of passive components, becomes relatively more expensive to realize as an integrated circuit than the present invention and would be somewhat bulky and more expensive to miniaturize than the present invention using other techniques for incorporation into cordless devices.
It is known from the further U.S. Pat. No. 3,970,912 issued on July 20, 1976 to Philip A. Hoffman and entitled "Battery Charging Circuit" to provide a battery charging circuit free of transformers and operatively arranged to produce current pulses which are supplied to the battery or batteries to be recharged via the inductance of an electric motor, which forms part of a cordless hand tool or the like. This circuit, while not requiring capacitors, does require at least two diodes and an inductance, albeit the inductance of an electric motor which is a portion of a powered hand tool or the like. As a result, this circuit has somewhat limited utility because of the requirement for an inductance, and, in particular, the inductance provided by a D.C. electric motor.
A considerable number of battery chargers have been proposed and are known from the general prior art including U.S. Letters Patents identified as follows:
______________________________________ Numbers Patentees Issue Date ______________________________________ 3,281,639 Norman N. Potter et al. October 25, 1966 3,735,233 Richard B. Ringle May 22, 1973 3,876,921 John H. Bigbee, III April 8, 1975 4,013,934 George J. Frye March 22, 1977 4,140,958 Charles R. Groeschel February 20, 1979 4,158,813 Robert W. Ellis et al. June 19, 1979 4,162,439 Arthur Schneider July 24, 1979 4,186,335 Harold J. Cahill January 29, 1980 4,220,905 William T. Quarton September 2, 1980 4,266,178 Tatsushi Asakawa May 5, 1981 4,292,578 Robert L. Steigerwald et al. September 29, 1981 4,321,523 Ronald O. Hammel March 23, 1982 4,348,619 Ray et al. September 7, 1982. ______________________________________
It is also known from Mims III "Engineer's Notebook A Handbook of Integrated Circuit Applications", First Edition, Second Printing, pg. 95, Radio Shack, A division of the Tandy Corporation, U.S.A. (1979) to use integrated circuits in battery chargers.